The present invention relates to a transmission case for tractors.
Generally farm tractors are required to have a wide range of traveling speed. They must run at high speed while driving on the road and at very low speed while digging ditches on farm land. For example, large general purpose tractors are required to meet the multiple speed variation capability, resulting in an especially long transmission case which cannot be used commonly with small tractors that do not require such a multiple speed variation capability.
To overcome the above inconvenience there is available a transmission case which, as shown in FIG. 1, is divided into a front and a rear transmission case 1 and 2 and has a spacer transmission case 6 mounted between the front and rear cases with a first speed change gear 3 contained in the front transmission case, a second speed change gear 5 in the spacer transmission case and a third gear 4 in the rear transmission case. For the tractors that do not require the second speed change gear 5 only the front and rear transmission cases 1 and 2 are coupled with the spacer transmission case 6 omitted. Hence, in the tractors that use the transmission case as shown in FIG. 1, the entire length of the transmission case or wheel base will vary according to the number of speed change gears required. This is disadvantageous in the light of standardization. The transmission having the spacer transmission case 6 will inevitably be long in the entire length in contrast to the general requirement for smaller size.
In this type of transmission, a hydraulic cylinder 7 and a piston 8 are provided in the upper portion of the rear transmission 2 for hanging the cultivating tools with a lift arm 9 linked with the piston 8. The hydraulic cylinder assembly is installed in a recess 2a formed in the upper portion of the rear transmission case 2. To keep the entire length of the transmission case as small as possible, the rear transmission case 2 has to be reduced by as much length as the spacer transmission case 6. To satisfy the above requirement while securing the length of recess 2a necessary for accommodating the hydraulic cylinder assembly makes the size (L) of the mounting portion 10 measured from the front end 2b of the rear transmission case 2 to the front edge of the recess 2a become smaller. As a result when the rear transmission case 2 is mounted on the front transmission case 1 through the spacer transmission case 6, vibration will result during operation due to insufficient strength of the mounting portion 10.